Cosmology and Astrophysics from Relaxed Galaxy Clusters II: Cosmological Constraints (1402.6212v2)

Abstract: We present cosmological constraints from measurements of the gas mass fraction, $f_{gas}$, for massive, dynamically relaxed galaxy clusters. Our data set consists of Chandra observations of 40 such clusters, identified in a comprehensive search of the Chandra archive, as well as high-quality weak gravitational lensing data for a subset of these clusters. Incorporating a robust gravitational lensing calibration of the X-ray mass estimates, and restricting our measurements to the most self-similar and accurately measured regions of clusters, significantly reduces systematic uncertainties compared to previous work. Our data for the first time constrain the intrinsic scatter in $f_{gas}$, $(7.4\pm2.3)$% in a spherical shell at radii 0.8-1.2 $r_{2500}$, consistent with the expected variation in gas depletion and non-thermal pressure for relaxed clusters. From the lowest-redshift data in our sample we obtain a constraint on a combination of the Hubble parameter and cosmic baryon fraction, $h^{3/2}\Omega_b/\Omega_m=0.089\pm0.012$, that is insensitive to the nature of dark energy. Combined with standard priors on $h$ and $\Omega_b h^2$, this provides a tight constraint on the cosmic matter density, $\Omega_m=0.27\pm0.04$, which is similarly insensitive to dark energy. Using the entire cluster sample, extending to $z>1$, we obtain consistent results for $\Omega_m$ and interesting constraints on dark energy: $\Omega_\Lambda=0.65^{+0.17}_{-0.22}$ for non-flat $\Lambda$CDM models, and $w=-0.98\pm0.26$ for flat constant-$w$ models. Our results are both competitive and consistent with those from recent CMB, SNIa and BAO data. We present constraints on models of evolving dark energy from the combination of $f_{gas}$ data with these external data sets, and comment on the possibilities for improved $f_{gas}$ constraints using current and next-generation X-ray observatories and lensing data. (Abridged)

Cosmology and Astrophysics from Relaxed Galaxy Clusters: Analysis and Implications

The investigation of massive, dynamically relaxed galaxy clusters presents an intriguing method for probing cosmological scales through measurements of the gas mass fraction (\fgas). This paper, the second instaLLMent in a series focusing on these galaxy clusters, explores the cosmological constraints derived from \fgas{} measurements while refining methodologies previously employed. The authors have significantly extended the dataset by utilizing X-ray observations from \Chandra{} and incorporating weak gravitational lensing data. By examining 40 galaxy clusters within this framework, the paper addresses systematic uncertainties and advocates for enhanced precision in modeling cosmological dynamics.

Key Findings and Methodological Advances

The paper highlights a sophisticated approach to reducing systematic errors by implementing three major methodological improvements:

1. Automatically Identified Relaxed Clusters: Through refined techniques based on morphological properties derived from X-ray imaging data, the authors automate the selection process of galaxy clusters. This is critical to minimize biases due to non-equilibrium and structural complexities.
2. Gas Mass Fraction Measurements in Spherical Shells: Measuring \fgas{} within spherical shells about $r_{2500}$ rather than cumulative to that radius significantly improves theoretical uncertainty by excluding disturbed central regions.
3. Gravitational Lensing Calibration: An advanced calibration utilizing weak gravitational lensing directly contrasts X-ray mass estimates, thereby reducing systematic uncertainties linked with non-thermal pressures or biases due to hydrostatic equilibrium assumptions.

Moreover, the analysis successfully constrains the intrinsic scatter in \fgas{} at $7.4 \pm 2.3\%$, a figure that reflects the coherent behavior of these relaxed clusters and aligns with expectations for relaxed clusters’ properties.

Cosmological Constraints

The cosmological implications drawn from the \fgas{} measurements emphasize constraints on key parameters such as the matter density (\Omegam) and the dark energy equation-of-state parameter (\w). For non-flat \LCDM{} models, the data provide estimates of $\Omegam = 0.29 \pm 0.04$ and $\Omegal = 0.65^{+0.17}_{-0.22}$. Notably, these constraints are competitive and consistent with those inferred from complementary probes like CMB, BAO, and type Ia supernovae.

Potential for Further Refinement

While the current sample exhibits commendable precision, gaining more relaxed clusters—particularly at redshifts exceeding $0.5$—is indispensable for strengthening dark energy constraints further. However, achieving substantial precision improvements necessitates next-generation observational paradigms along with continuous advancements in gravitational lensing techniques. The projection of potentially observing hundreds of clusters with next-generation X-ray observatories (NXO) like SMART-X or ATHENA+ promises significant constraint enhancements.

Conclusion

This paper offers valuable insights and robust constraints that inform cosmological models while setting the stage for future observational opportunities. Extending the framework to incorporate a broader array of cluster observations, validated by high-quality lensing calibration, affirms its potential to elucidate complex cosmological phenomena. The methodological rigor and promising prospects underscored in the paper pave pathways for more refined explorations at the forefront of cosmological research.